1. Field of the Invention
The invention relates to a method for producing a rotationally symmetrical molded part of a high-temperature superconductor.
2. Discussion of Background
DE-A1-4,019,368 discloses a method is described for producing cylinders or rings on the basis of a bismuth two-layer cuprate. In this case, the homogeneous melt is spun into a rotating, cold fusion mold. Very different solidification rates at the edge and in the interior of the specimen produce a microstructure having a very different density and full of internal stresses. Nothing is reported on the values of the critical current density.
EP-A1-0,482,221 discloses a method for producing a ceramic high-temperature superconductor of the Bi-Sr-Ca-Cu-O type, in which the ratio of Bi:Sr:Ca:Cu is (2-x):2:(1+v):(2+w), where 0.05.ltoreq.x.ltoreq.0.6; 0&lt;v.ltoreq.0.5 and 0&lt;w.ltoreq.0.5. Oxides of Bi and Cu and carbonates of Ca and Sr are converted below the melting temperature, preferably at 800.degree. C., into a Bi 2-layer compound. The powder mixture is then pressed into a desired shape and melted a few degrees above the melting temperature for 1 h at 1 bar O.sub.2. In a two-stage process, (13 h at 860.degree. C. at 1 bar O.sub.2 and 60 h at 800.degree. C. in air), the material is aftertreated and converted into the Bi 2-layer compound. The subsequent cooling was carried out below 700.degree. C. in an N.sub.2 atmosphere. In the absence of a magnetic field, the critical current density j.sub.c was 1.6 kA/cm.sup.2. j.sub.c (B=0) was measured as 2.12 kA/cm.sup.2 for x=0.05 and an aftertreatment of 18 h at 800.degree. C.
It has been disclosed in EP-A2-0,351,844 to produce a high-temperature superconductor of the Bi-Sr-Ca-Cu-O type with a critical temperature of 94K and a critical current density of 2 kA/cm.sup.2. After heat treatment for 30 minutes in an oxygen atmosphere at 900.degree. C., heating was carried out for 10 minutes to 920.degree. C. and annealing was subsequently carried out for 6 h at 880.degree. C. Thereafter, cooling was performed at a cooling rate of 2K/min to 600.degree. C., and subsequently in a nitrogen atmosphere down to room temperature.
Jun-ichiro Kase et al., Partial Melt Growth Process of Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x Textured Tapes on Silver, Japanese Journal of Applied Physics, Vol. 29, No. 7, July 1990, pages L1096-L1099 has disclosed achieving a critical current density j.sub.c of 13 kA/cm.sup.2 for orientated thick films at a temperature of 77K without an external magnetic field.
A disadvantage of this method is the restriction to film thicknesses of &lt;100 .mu.m.
As in the case of the method according to the abovementioned. EP-A2-0,351,844, here, as well, the powder, pressed or poured, is applied to a planar substrate, preferably made from silver. Under the action of gravity in the partially melted state, the powder is compressed to form a compact body or film. In the case of non-planar, e.g. cylindrical substrates, this method fails due to non-uniform running or running out of the liquid.
The article by G. Triscone et al., Variation of the superconducting properties of Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x with oxygen content, Physics C 176 (1991), pages 247-256, particularly FIG. 1 on page 248, discloses how the critical temperature T.sub.c depends on the oxygen partial pressure in the case of equilibrium heat treatment. It is therefore possible to optimize Bi2212 superconductors of any type of manufacture and size with respect to the critical temperature.